The present invention relates to a broadband circularly polarized (CP) patch antenna. More particularly, it relates to a broadband circularly polarized patch antenna with a probe feed placed coplanarly with the radiating metal patch. Therefore, the inductance effect caused by a longer probe feed in thicker medium, such as air, will be decreased, and a circularly polarized patch antenna with the property of broadband operation, high gain, low cost and simple structure can be obtained.
To follow the advancement of the communication technology, the applications using communication technologies have been increased significantly and the related products have become more diversified. The design and study of antenna is more important, because an antenna is used to receive or deliver signals in communication products. In wireless communication, the properties of broadband operation and circular polarization are among the mainstream for the antenna design. Broadband operation can increase the transmission capacity and the transmission speed, and the property of circular polarization can decrease or avoid the multi-path reflection interference from the ambiance. Therefore, in wireless communications, the antenna with the features of broadband operation and circular polarization can be found in many applications, especially when the antenna has a high gain and can be constructed with low cost.
Referring to FIG. 1, FIG. 1 shows a 3D diagram of the structure of conventional rectangular patch antenna with a thick air substrate. In FIG. 1, a probe feed 20 of the conventional rectangular patch antenna with a thick air substrate (reference antenna) is connected with a radiating metal patch 25 from a ground plane 10 through a substrate (such as an air substrate) 15 that is between the radiating metal patch 25 and the ground plane 10, and a signal is fed to the radiating metal patch 25.
In order to obtain an antenna with high gain and broadband operation, the conventional method is to increase the thickness of the substrate 15, so that the quality factor of the antenna will be decreased to increase the radiation efficiency and the operating bandwidth of the antenna. Referring to FIG. 2, FIG. 2 is a diagram showing measured return loss of the conventional reference antenna (the center frequency is 1800 MHz). The dotted line 70 shown in FIG. 2 is a reference line indicating a 14 dB return loss or 1:1.5 VSWR (Voltage Standing Wave Ratio). The curve 50 indicates the impedance bandwidth that is measured from the reference antenna with 3 mm of the thickness of the substrate. The curve 55 indicates the impedance bandwidth that is measured from the reference antenna with 6 mm of the thickness of the substrate. The curve 60 indicates the impedance bandwidth that is measured from the reference antenna with 9 mm of the thickness of the substrate. The curve 65 indicates the impedance bandwidth that is measured from the reference antenna with 13 mm of the thickness of the substrate.
The impedance bandwidth of the antenna increases with the increase of the thickness of the substrate 15. However, as shown in FIG. 2, the return loss of the conventional reference antenna with 6 mm of the thickness of the substrate 15 is better than that with 9 mm and 13 mm of the thickness of the substrate 15, because a longer probe feed 20 is required for transmitting signals to the radiating metal patch 25 when the thickness of the substrate 15 increases. Therefore, the inductance effect caused by the longer probe feed 20 increases, because the probe feed 20 is connected with the radiating metal patch 25 through the substrate 15. Thus, the impedance matching is degraded, and the operating bandwidth of the antenna will be decreased.
In the other way, there are two known methods to achieve circular polarization operation. One is a single-feed method, and the other is a dual-feed method. However, for a conventional single-feed circularly polarized patch antenna, the 3-dB axial-ratio circular polarization bandwidth is not easy to be 3% above; i.e., the operating bandwidth of the aforementioned antenna is narrow so that its practical applications are limited. For a dual-feed circularly polarized patch antenna, a better 3-dB axial-ratio circular polarization bandwidth can be obtained; i.e., the operating bandwidth is wider, but it needs an external phase shifter circuitry, which makes the antenna design complicated and also increases the construction cost of the antenna. Therefore, in order to resolve the aforementioned problem, a circularly polarized patch antenna with high gain, wide band, low cost and simple design has to be provided.
In view of the background of the invention described above, the inductance effect caused by the long probe feed of the conventional reference antenna affects the impedance matching of the antenna. Moreover, the bandwidth of the conventional single-feed circularly polarized patch antenna is narrow, and the design of the conventional dual-feed circularly polarized patch antenna is complicated and the construction cost is high. Therefore, the conventional circularly polarized patch antenna does not have the features of low cost and wide operating bandwidth, so that the applications thereof are limited.
It is the principal object of the present invention to provide a broadband circularly polarized patch antenna. By using a probe feed placed coplanarly with the patch to convey signals directly to the radiating metal patch, the inductance effect caused by the long probe feed in the thick substrate can be decreased, and the impedance bandwidth can be increased. Through the study data, it is known that the broadband circularly polarized patch antenna of the present invention has the features of low cost, high antenna gain, wide operating bandwidth and good CP radiation, thereby overcoming the disadvantages of the conventional circularly polarized patch antenna.
In accordance with the aforementioned purpose of the present invention, the present invention provides a broadband circularly polarized patch antenna. The broadband circularly polarized patch antenna of the present invention consists of: an L-shaped ground plane; a radiating metal patch; a probe feed placed coplanarly with the radiating metal patch used to connect with the vertical ground plane and the radiating metal patch; and a substrate. In the broadband circularly polarized patch antenna of the present invention, the signal is directly fed to the radiating metal patch by using the probe feed placed coplanarly with the radiating metal patch, and the probe feed does not pass through the thick substrate so that the probe feed can have a smaller length, which decreases the probe inductance and makes better impedance matching easy to obtain. Moreover, the broadband circularly polarized patch antenna of the present invention has the features of high antenna gain, wider operating bandwidth, good circular polarization radiation and simple structure, so that the present invention is a valuable implementation in industrial fields.